And if you dug it up as a fossil, how would you know that it had big tail flippers? Or how would you know that it didn’t have (for instance) an elephant-like trunk?

Well, you would see a big powerfully built tail, with lots of attachments for strong muscles, so I think you’d be able to infer a set of flippers. At the other end, you’d see a set of nasal gear at the top of the skull, with no muscle attachments for an elephant-like trunk, implying lack of same.

And if you dug it up as a fossil, how would you know that it had big tail flippers? Or how would you know that it didn’t have (for instance) an elephant-like trunk?

Even from the photo you can see that the tail is sturdy and ends abruptly, suggesting something massive was attached. I am not an anatomist, but I imagine the joints between the tail vertebrae would permit up and down movement but not much from side to side. There may be other possibilities, but the only one I can think of that would result in this type of structure (presuming it is not flexible enough to behave like a giant springtail) would be large tail flukes.